1. Field of the Invention
The present invention relates to a method for generating a frequency reference signal particularly for a wavelength-division multiplex operation, and to an arrangement for executing the method.
2. Description of the Related Art
The continuously growing bandwidth requirement in telecommunications can be satisfied in the core network by applying wavelength-division multiplexing. This technique enables a better exploitation of the inherent transmission bandwidths of light waveguides based on quartz glass than the single-channel transmission common today. Light sources which emit their light at a prescribed frequency with a high degree of precision and optical filters whose transmission function is adapted to the selected channel raster are preconditions of a successful application of the wavelength-division multiplex technique. Since the properties of both laser diodes a employed as light sources, and suitable optical filters are dependent on temperature and undergo aging processes, it is a requirement to check the frequency precision and, given the great demands of the wavelength-division multiplex technique, control the emission frequency of laser diodes or the midband frequency of the optical filter. Automatic transitions, or molecular resonances of the krypton 84 isotope, of ammonia, acetylene, or hydrocyanic acid, in particular, are used for this, in view of the high precision of the frequency reference (Chung, Y C, Clark, L, Fan, C, xe2x80x9cStandardization of Optical Frequencies for WDM Networking Applications,xe2x80x9d Laser Frequency Stabilization and Noise Reduction, 1995; SPIE, Vol. 2378 (Int. Soc. Opt. Eng., San Jose: 230-235)). The transition between 1s2 and 2p8 of the krypton 84 isotope, which corresponds to a frequency of 193.68622 THz, is considered particularly suitable for the synthesis of a frequency reference in the third optical window, i.e. at 1550 nm. The article xe2x80x9cAbsorption Spectra of Excited Kr84 States Between 1.5 and 1.58 xcexcm and Their Use for Absolute Frequency Lockingxe2x80x9d (von Helmolt, C., J. Lightwave Techn., Fischer, U.H.P.; Vol 14, No. 2 (1996):139-143) described an arrangement in which the collimated beam of a laser diode LD1 is directed through of a galvanotron which is filled with krypton 84, which galvanotron is connected to a high-voltage source via a resistor. The laser diode is modulated with a signal of small amplitude and low frequency, a synchronous signal thereby emerging at the resistor, from which signal a control signal is derived by means of synchronous detection, which control signal adjusts the emission frequency of the laser diode to the value of the atomic transition, thereby generating a first reference signal. However, a problem derives in that the reference signal does not fit into the channel raster recommended by the ITU-T (xe2x80x9cInternational Telecommunication Union-Telecommunication Standardization Sectionxe2x80x9d).
It is an object of the Invention to provide a method and an arrangement for generating an optical frequency reference signal which lies in the ITU raster.
This object is achieved by providing a method and an arrangement in which a method includes the steps of generating a first optical signal via a first laser diode in which a low-frequency signal of small amplitude is superimposed on the first optical signal, conducting the first optical signal through a gas-filled galvanotron, the low-frequency signal emerging at two terminals of the galvanotron, feeding the low-frequency signal to a synchronous detector and emitting a first output signal for generating a control signal, controlling the first laser diode via the control signal and the low-frequency signal, wherein the first optical signal passing through the gas-filled galvanotron represents a first frequency reference signal, generating a second a optical signal via a second laser diode, emitting the second optical signal via a first fiber-optic directing coupler to a signal output and to a second fiber-optic directing coupler, generating a microwave signal by combining the second optical signal and the first frequency reference signal via an opto-electronic converter, feeding the microwave signal to a frequency control loop subsequent to opto-electric conversion and mixing the microwave signal with a quartz-based frequency synthesizer signal for generating a difference low-frequency signal, and providing a phase comparison of a signal of a quart oscillator and the difference low-frequency signal resulting in a second output signal for generating a control signal for the second laser diode.
In an embodiment of the invention in which the microwave signal is a first microwave signal, the method further includes the steps of controlling the output of the second laser diode by generating the second optical signal that is within a preselected frequency channel raster, and generating a second microwave signal that differs from the first microwave signal by a few 100 MHZ.
In an embodiment of the present invention a circuit arrangement is provided for generating a frequency reference signal that includes a sub-circuit for producing a first frequency reference signal, the sub-circuit has a first oscillator, a synchronous detector, a first controller, an adder, a first laser diode, a galvanotron, and first and second lens, wherein the first oscillator has a first output connected to a first input of the synchronous detector and the second output connected to a first output of the adder generates a low-frequency signal, the synchronous detector also has second and third inputs connected to the galvanotron via a high voltage source, a capacitor and a resistor and an output connected to the first controller, the first controller having an output connected to a second input of the adder generates an output signal to the adder, the adder generates a control signal to the first laser diode via a first laser diode current source, the first laser diode, galvanotron and first and second lens connect to form a light path that outputs the first frequency reference signal. The circuit arrangement also includes a second current source, a second laser diode that is connected to the second current source in which the second laser diode generates an output light, a third lens, a first fiber-optic directing coupler that receives the output light via the third lens the first fiber-optic directing coupler also has a first input and first and second outputs, the first output is connected to a signal output via a third optical fiber, a second fiber-optic directing coupler having first and second inputs, the first input being coupled to the second lens via a first optical fiber, the second input connected to the second output of the first fiber-optic directing coupler via a second optical fiber, an opto-electronic converter connected to the second fiber-optic directing coupler, the opto-electronic converter has an output and generates a first microwave signal, a modulator having first and second signal inputs and an output in which the first signal input is connected to the output of the opto-electronic converter, a frequency synthesizer has an input and an output that is connected to the second signal input the frequency synthesizer generates a second microwave signal, a quartz oscillator has first and second outputs in which the first output is connected to the input of the frequency synthesizer, a first frequency divider includes an input that is connected to the modulator via a low pass filter, the first frequency divider also having an output, a phase detector included first and second inputs and an output in which the first input is connected to the output of the first frequency divider, a second frequency divider has an input connected to the quartz oscillator and an output connected to the second input of the phase detector, and a second controller that has an input connected to the output of the phase detector and an output connected to the second current source, wherein the second controller generates a control signal to the second current source and the second laser diode generates a second frequency reference signal.
In an embodiment, the circuit arrangement includes connections via the first, second and third optical fibers to and from the first and second fiber-optic directing couplers maintain polarization.
In an embodiment, the circuit arrangement the first and second laser diodes are DFB lasers.
It is an advantage of the invention to operate only a few components in the microwave region, while the majority are passed only by a signal with a considerably lower frequency.
The invention is detailed below with the aid of an exemplifying embodiment.